The mechanical properties of ferrous based powder metallurgical components are density limited. In general, the higher the density at any given alloy content, the higher the resultant properties. Consequently, in order to increase mechanical properties without resorting to high alloy content with minimal increase in cost, the major thrust of research in ferrous powder metallurgy in the last quarter century has been to increase density. Traditionally, compaction and sintering techniques have been used to increase density. Of the two, compaction has received the most attention.
In general, densification of a metal powder by compaction involves two different processes. At low pressures, densification occurs as a result of a re-packing process whereby the particles of the powder slide and/or rotate past one another into juxtaposed points of minimal or near minimal spacing. Thereafter, at higher pressures, densification occurs as a result of in situ plastic deformation of individual particles.
The density achieved by conventional compaction techniques depends on the powder composition of interest. Two factors that affect the maximum achievable density of a powder metallurgy composition are lubricant content and the compressive plastic flow properties, or so-called compressibility, of the base powder. Typically, the maximum achievable density increases as the compressibility of the base powder increases.
Lubricants facilitate ejection of compacted parts from a die by lubricating the die wall, lubricants and also assist the re-packing process by lubricating the particles of the powder. The lubricated particles slide and/or rotate past one another with greater ease compared to non-lubricated powders. Lubricants, however, also interfere with densification during the plastic deformation process. In particular, as deformation occurs, the lubricant concomitantly extrudes into and eventually fills the remaining pore spaces within the compact. Whereupon, since the lubricants are typically amorphous materials and essentially behave as an incompressible fluid, the lubricants often prevent further collapse of pore spaces, in effect, impeding densification.
Therefore, the powder metallurgy industry has traditionally sought to increase the compressibility of the base powder and minimize the lubricant content needed to meet the ejection requirements without adversely effecting the powder's ability to densify during the re-packing stage of compaction. For example, U.S. Pat. No. 5,154,881 to Rutz and Luk and U.S. Pat. No. 5,368,630 to Luk describe warm compaction technologies, which permit the use of lower compaction temperatures and lower lubricant contents. Unfortunately, warm compaction processes, like all compaction-based approaches to densification, are limited by the compressibility of the compacted composition.
Another drawback to densifying parts by compaction is that compaction is normally non-isotropic thereby resulting in density gradients within the body of the part. Consequentially, the final dimensions of the part are difficult to control due to shrinkage, which is a function of local density.
Like compaction techniques, sintering processes also densify compacted parts. However, significant densification by sintering is limited by the difficulty of controlling the final dimensions of the part. In addition, it has the practical drawback that it can only be achieved by the use of high sintering temperatures, which require high temperature furnaces that are expensive to purchase and operate.
Double press and sinter processes are another traditional technique for achieving higher densities. For this method, a metal powder is compacted and submitted to a combination lubricant burn-off and inter-critical anneal at a low temperature, for example, in the ferrite to austenite transformation range, (i.e. from about 1355 to about 1670° F.). Thereafter, the compacted part is compacted a second time, and finally sintered at a relatively higher temperature in the austenitic range, (e.g. typically, at about 2050° F. in a production belt furnace). As with other sintering processes, the extra compaction and sintering steps adds significantly to the cost of powder metallurgy parts. Moreover, the maximum achievable density is limited in double press and sinter process due to the natural decrease in compressibility of the compacted part during the second compaction step.
Conventional infiltration techniques are also used to fabricate high density ferrous based parts using a non-ferrous material such as copper or, an alloy of copper. These techniques are limited metallurgically, however, by the use of copper. In addition, use of copper typically adds more to the costs of fabricating powder metallurgy part than conventional double press and sinter techniques.
Therefore, manufacturers continually seek powder metallurgy techniques for preparing compacted parts with desirable mechanical properties and high density at low cost. Hence, methods and compositions that satisfy these requirements are desired.